drm/amdgpu: add mmhub v9.4.1 block for Arcturus (v2)

Arcturus as an updated mmhub block. mmhub is the
memory controller hub used for sdma and multimedia.

v2: squash in AGP BAR programming (Alex)

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>